My wife and I recently found out that we are going to have twins and so nearly everyone asks if we have a family history of twins. Now I know that the answer for me is that it doesn't matter—as a man, I have no influence on my wife's tendency to hyper-ovulate. And, of course, if we turn out to have monozygotic twins (we don't know yet), genetics plays no part.

Therefore the question only matters on my wife's side and only if dizygotic twins are known in her heritage. As it happens, we don't know of any twins on her side. Given that many factors besides genetics are relevant to twins being born and we only know her family history for a half-dozen generations with any confidence, what are the odds that we would observe twins in her family history if she is genetically predisposed to having them?

Or, to put in another way, can I respond that nutrition and other factors were probably more important than genetics?

However, because fraternal twins can be the result of a gene on the X-Chromsome, the answer is yes in this specfic case. http://genetics.thetech.org/ask/ask86 If a mother is, herself, a fraternal twin then her chances of bearing fraternal twins is 2.5x higher than normal and it can be inherited from the mother's side even if she's not a twin herself. It's definitely not guaranteed, as the normal chances are so low to begin with, but there is definitely an increase.

However, for all other types of twins I have not yet been able to find anything implying there's a genetic component.

Hmm... I read those references as well and my impression was that nutrition did have a significant effect. The age factor sounded more like a function of fertility treatments than age itself. (We were actually trying to hold off on more children as it happens--our birth control failed.) It does seem like a reasonable thing to point out is that my wife is not herself a twin. But I'm more curious about the question of whether searching back through the family tree is a waste of time, statistically speaking.
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Jon Ericson♦Aug 27 '12 at 19:49

I am still catching up on this topic but there are (atleast) 3 documented towns with greater than average rate of twinning: Cândido Godói, Kodinhi and Igbo-Ora. A published study on the Brazilian town might also be of help. It might be informative to look at familial twinning in these examples as beginning clues.
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gkadamAug 27 '12 at 20:10

Do you have a source for: The propensity for heterozygotic twins seems to be driven primarily by genetics?
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Jon Ericson♦Aug 27 '12 at 22:20

The Nature site is working for me. An interesting quote: "Intra-uterine selection may be responsible for the Hardy−Weinberg deviation and we may in fact be dealing with a gene involved in the intrauterine survival of dizygotic twins." I'm not sure I understand that, but it sounds like this study doesn't link genetics to hyper-ovulation, but survival in the womb. Most of the information I've read so far point to a genetic link to the number of eggs released instead.
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Jon Ericson♦Aug 27 '12 at 22:23

@JonEricson, added a couple of sources. It is my own impression from reading the sources that genetics is the most important. One way to think of it - without the genes for it, there is virtually 0 chance of it happening. In other words, it doesn't seem possible to just eat a bunch of soy beans, let's say, and have twins as a result.
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Lubo AntonovAug 27 '12 at 22:34

@JonEricson, I don't think they try to establish any kind of causal relationship between the gene and hyperovulation. They simply found a suggestive correlation, but for the purposes of your question that would indicate a strong genetic component, I think. The study was geographically localized, which makes it more difficult to reach a general conclusion. The interesting thing that I noticed: they throw some doubt on the 'maternal lineage only' hypothesis. The quote you mentioned, I think it simply gives an alternative explanation for the disequilibrium in the alleles that they found.
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Lubo AntonovAug 27 '12 at 23:02

I think it would help if some of your comments were added to the answer. I particularly appreciate: "it doesn't seem possible to just eat a bunch of soy beans, let's say, and have twins as a result." On the other hand, the practical aspect of my question is really a statistical one. Are the genes for twinning very rare or do they just rarely produce twins?
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Jon Ericson♦Aug 28 '12 at 18:33